A stability and control system for a hexapod underwater robot

Aqua is an underwater hexapod robot that uses paddles to propel and orient itself. The system is highly non-linear and coupled, and thus far a controller has not been implemented on the robot. In this work, three different controllers were developed and utilized on the robot. The design of a controller for the vehicle began with the development of a stability augmentation system (SAS). In order to study the stability of the system, the model needed to be linearized and this was accomplished using numerical differentiation by finite differences. Using this method state space matrices were derived for three different steady state velocities and corresponding SAS's were designed based on the system's eigenvalues. These SAS's were implemented in a nonlinear simulation and were shown to need further refinement. The refined SAS's were then designed and were successfully implemented on the physical robot in fresh and sea water. The design of an autopilot to operate with the SAS followed which included a proportional and a proportional-integral controller. The controllers were tested in simulation and in experiment with inconsistent results. Finally, the SAS was modified to compensate for possible faults that may occur during the operation of the robot. It was found that the original SAS was sufficiently robust to compensate for the case of a missing flipper. However, the case of a flipper stuck at a fixed angle required a modification to the SAS and this was accomplished by analyzing the additional drag forces created by the fault. The modified SAS was implemented on the robot in a set of experiments with successful results.